That is an interesting question, I've often wondered what other churches believe on this subject. To my mind, the answer seems pretty simple. If the Bible is the only word of God, then we are to believe that God has never, and will never say anything else. Ever. Right?  
 
So, I think about my life, prayers that have been answered, understandings that have come to my mind. Things that I've known, important things, that I know came to me from God, because I could not have received them any other way.  
 
In those moments (and the more I think about them, the more I seem to find)I believe that God has spoken to me. That he has sent his spirit to be with me. And then I go home, and I write the things that I have learned. It helps me to remember, and to understand more clearly the things God wants from me.  
 
I believe that I am not unique in this.  
 
So what does that mean about the Bible? That it isn't true? Of course not. Does it mean that my inspiration couldn't have come from God? No. And what if someone else were to read my journal, and feel that same spirit, and learn something new, or understand something differently in a way that draws them nearer to the Lord? Does that make them sacrilegious? No. 
 
To me, it means that God still loves us. God still teaches us. God still wants us to succeed, that he has always been, and will always be, an active figure in our lives, and in our salvation.  
 
That is what modern revelation is about. And yes, the Book of Mormon is a big part of that. Not because it's all different from the Bible, and not because it's all the same, but because it works together with the Bible to reinforce the things that are most important to the Kingdom of God, and to clarify things that may have been misunderstood because of translation, or incorrect traditions, or from Satan being just plain tricky.  
 
The Book of Mormon works hand in hand with the Bible to clarify God's plan for us, and to teach us how to perfect our lives and draw closer to Him. And ultimately return to be with Him again. 
 
I know the verse in Revelations 22:18, that says:  
 
"If any man shall add unto these things, God shall add unto him the plagues that are written in this book" 
 
But it seems to me that there are at least two different ways to understand it. One way, is to say that the Bible is it. God will never say any more. Period. But I like the other way better. To me, it reads like this: 
 
God endorses this book. It is scripture. It is sacred. And anything you do to try to change it, (leave out the parts you don't like, or re-word it so that God's teachings line up more with your personal beliefs) is unfathomably wrong, as it will lead generations after you astray. We should seek to change our will to be more like Gods, not try to change God's words to our way of thinking. No matter how good or bad our intentions are, the only one who has the right to clarify God's teaching, is God. And by extension, the person to whom God entrusts that work.  
 
So, yes, the Bible is good. But God has so much more in store for us. So many great blessings he wants us to have, (including everything the Book of Mormon has to offer). But if you tell God, "I'm done listening," then you limit what he can do for you.